This invention relates to a process for the manufacture of calcium fluoride and more particularly to its production by the reaction of fluorosilicic acid (i.e., FSA or aqueous H2SiF6) with phosphate rock containing fluoroapatite.
Fluorine, the essential element for fluorochemicals and fluoropolymers, is currently derived from fluorspar, a mineral which is a crystalline form of calcium fluoride. Reserves of fluorspar are rapidly being depleted. Furthermore, the United States currently imports about 90% of its supply.
An important reserve of fluorine is contained in fluoroapatite, i.e., CaF2.3Ca3(PO4)2, a mineral which is used for the manufacture of phosphoric acid. This mineral constitutes a fluorine reserve which is about four times greater than the proven reserves of fluorspar. During the manufacture of phosphoric acid from fluoroapatite, most of the fluorine is removed as fluorosilicic acid (FSA). There is some demand for FSA for fluoridating drinking water and for the manufacture of cryolite and aluminum fluoride. However, since this demand is small, most of the fluorine produced during phosphoric acid manufacture is sent to a waste-water pond. This can cause a fluorine pollution problem.
Over the years numerous processes, several of which are described below, have been developed to recover the fluorine from phosphate minerals. The United States Bureau of Mines (Chem. Abst., 75:23270, 1971) has shown how waste fluorosilicic acid can be converted to an acid-grade fluorspar (CaF2). A first step involves treating the FSA with ammonia to precipitate silica, which is removed by filtration, and form NH4F. In a second step ammonium fluoride is treated with lime to form CaF2.
U.S. Pat. No. 5,531,975 describes a process for reacting phosphate rock and FSA to produce a slurry comprising phosphoric acid, calcium fluoride, silicon dioxide and undigested phosphate rock. An excess stoichiometric amount of calcium to fluorine is initially present in the slurry. The product slurry is pumped into a vacuum filter or centrifuge where the phosphoric acid and colloidal calcium fluoride are separated from the undigested phosphate rock and silica. In Example 1, the weight ratio of F:Si in the product (initial filtrate) is shown to be about 30:1.
We have discovered a process which, surprisingly, is more efficient for producing calcium fluoride from fluorosilicic acid and with a much higher recovery of fluorine from the acid than previously reported. The calcium fluoride produced also contains much lower levels of silica. Moreover, the process may be run in a continuous manner.
This invention provides a process for producing calcium fluoride comprising:
(a) mixing phosphate rock with aqueous H2SiF6 at ambient temperature for at least one hour;
(b) nucleating the calcium fluoride in the slurry prepared in step (a) by reacting the slurry at a temperature and for a time sufficient to initiate and sustain nucleation of calcium fluoride and SiO2 by-product;
(c) aggregating the SiO2 by-product by heating the slurry produced in step (b) at a temperature of about 90xc2x0 C. to about 105xc2x0 C. for at least 0.5 hours;
(d) recovering a calcium fluoride-containing product of an average particle size of less than 1 micron.
The phosphate rock useful in the current process is any naturally occurring phosphate rock and typically is comprised primarily of tricalcium phosphate (Ca3(PO4)2), calcium carbonate (CaCO3) and calcium fluoride (CaF2). This phosphate rock may also be used for the manufacture of phosphoric acid. The phosphate rock is preferably used as a rock powder or as a water-slurried rock powder.
The concentration and source of aqueous H2SiF6 used in this process is not critical. The aqueous fluorosilicic acid (FSA) produced by a phosphate plant may be used, which is typically 20 to 30% by weight H2SiF6. By FSA is meant a solution of 20 to 30% by weight H2SiF6 in water.
The reaction of phosphate rock with fluorosilicic acid can be carried out in a single reactor, more preferably in two reactors connected in series or most preferably in three reactors connected in series. Surprisingly, it has been found that the conditions required for optimum rock dissolution in fluorosilicic acid, nucleation of CaF2, and nucleation and growth of the SiO2 by-product are sufficiently diverse that the reaction is optimally performed in three separate reactors connected in series.
In a single reactor embodiment, dry phosphate rock powder or water-slurried phosphate rock powder and FSA feed solution (prepared by diluting aqueous H2SiF6 with water and optionally adding a surfactant) are continuously co-fed to the reactor. Product slurry is continuously removed. Reaction conditions are typically a reaction temperature of about 100xc2x0 C. with a residence time of 1 to 2 hours.
In a two-reactor embodiment, a batch feed tank feeding a continuous stirred tank reactor (CSTR) is used. In this mode dry phosphate rock powder or waterslurried phosphate rock powder and FSA feed solution are added batchwise to a large stirred tank at ambient temperature. The residence time is such that the calcium fluoride product is nucleated, typically at least 10 hours. The slurry from the batch feed tank is fed continuously to a CSTR. The CSTR is typically maintained at about 100xc2x0 C. with a residence time of 1 to 2 hours.
In a three-reactor embodiment, phosphate rock powder or water-slurried phosphate rock powder (preferably the slurry concentration is equal to or greater than about 70% solids by weight) and fluorosilicic acid are continuously co-fed to the first reactor, with continuous stirring at ambient temperature. Residence times in this reactor can vary between 1 to 36 hours depending on the water content and the reactivity of the rock feed.
The contents of the first reactor are continuously pumped to the second reactor. At the beginning of the run, the second reactor is heated to a temperature sufficient to initiate nucleation of calcium fluoride, typically about 75xc2x0 C. to about 90xc2x0 C. After nucleation has occurred, the reactor may be cooled to a lower temperature sufficient to sustain nucleation, typically 30xc2x0 C. to 50xc2x0 C. Alternatively, nucleation can be initiated by addition of HF to the reaction slurry or by use of a previous run""s product slurry which contains calcium fluoride. The residence times are typically between about 5 minutes to about 2 hours.
The contents of the second reactor are continuously pumped to the third reactor, also a CSTR. The third reactor is maintained at a temperature of about 90xc2x0 C. to about 105xc2x0 C. throughout the reaction, with a residence time of about 0.5 to 3 hours. In this reactor the silica by-product is agglomerated to a particle size which will facilitate separation of the colloidal calcium fluoride product from the silica, typically about 10 microns. The contents of the third reactor are continuously pumped to product collection.
In another embodiment of a three-reactor system, the second CSTR reactor is replaced by a plug-flow type reactor, e.g., a pipe or a coil constructed of a material compatible with both the reactants and products such as steel, copper or Teflon(copyright) (polytetrafluoroethylene) heated at about 100xc2x0 C., with a residence time of a few minutes.
From the above description, it can be seen that in the three-reactor embodiment, the temperature of the second reactor can be between about 30xc2x0 C. to about 100xc2x0 C.
The calcium fluoride product contains CaF2 microcrystals embedded in an amorphous calcium phosphate matrix. The as-recovered product is a slurry of solid calcium fluoride product suspended in about 32 wt. % aqueous phosphoric acid. The molar ratio of calcium fluoride to phosphate in the washed, dried solid is from about 2 to 10. The ratio depends on the concentration of the phosphoric acid in the slurry, the extent of washing of the dry solid calcium fluoride product and the solvent used. In a second step excess sulfuric acid is mixed with this suspension or with the dried product and heated to about 1 00xc2x0 C. to produce hydrogen fluoride, preferably containing less than about 36 wt. % water.
In still another embodiment, the calcium fluoride product may be separated from the phosphoric acid by mixing the suspension with an organic solvent that is miscible with phosphoric acid, such as methanol, ethanol or isopropanol, to reduce the calcium fluoride product solubility and to increase the product separation rate.
The calcium fluoride product may be recovered by any solid-liquid separation technique such as filtration, decantation or centrifugation. Separation may be done using the hot slurry, or the mixture may be cooled prior to the separation. The reaction and separation steps can be operated either in batch or continuous modes. Recovery may include washing using standard techniques (e.g., with water following initial filtration, centrifugation or decantation). The calcium fluoride produced using this invention has a F:Si weight ratio of greater than about 50:1, preferably greater than about 99:1.
The process of this invention provides a means for producing a calcium fluoride product which does not require the elaborate silica separation steps described in the art since the calcium fluoride product is colloidal (i.e., its particle size is less than about 1xcexc) and remains suspended in the aqueous phosphoric acid by-product while the silica as well as other contaminants in the rock settle out.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following embodiments are, therefore, to be construed as merely illustrative, and do not constrain the remainder of the disclosure in any way whatsoever.